1. Field of the Invention
The present invention relates to devices for monitoring tires. More particularly, it relates to a device which is simple and not very expensive, for warning of the under-inflation of a tire.
2. Description of the Background Art
Many devices have been offered to warn the driver of a vehicle of a pressure drop inside one of his tires. Most of these devices use sensors placed in the rim of the wheel for measuring pressure and/or temperature. These sensors transmit their measurements continuously, or when a threshold is crossed, to a central control package. Such devices are described in applications WO 87/00127, DE 2 923 258 or U.S. Pat. No. 4,052,696, for example. The necessity, in all these devices, to reliably transmit the measurements of the sensors located in the rotating wheel to the chassis of the vehicle, leads to high-cost systems reserved for high-price vehicles.
A simpler device is described by patent application DE 3 541 494. Its principle consists in using, to warn of the pressure drop, the change of the vibratory behavior of the wheel when the inflation pressure of the tire decreases, by means of a vibration sensor placed in the vicinity of the wheel-carrier. Unfortunately, the complexity, and the gradual nature of the change in this vibratory behavior of the wheel with the inflation pressure of the tire, causes a low sensitivity of this device. Moreover, its calibration varies as a function both of the type of tire and the type of vehicle.